Considering WoW was really my first PC game, as well as MMO (back in Vanilla). I'm really embarrassed about:
-Played a shaman during Vanilla, and raided as well with a pretty decent guild. I never figured out that you could target a person to heal, prior to casting the heal on them. I would queue up the heal, and then find that person to click on them, to cast the heal.
-When I swapped to my rogue for BC, I had no concept of what 'Hit Rating' was, until a good friend helped me out. I guess being a healer is a good excuse for that one.
-I used to not train skills from my class trainer, for skills I didn't think I needed.
-I used the shaman Far Sight bug in Arathi Basin to get 5 cap BG wins, before the BG ever started.
-I fell for the 'If you look through the window behind Golemagg, you can see Ragnaros!' prank.... more than once.
-I passed on Baron Rivendare's mount in Vanilla, because I thought that since I already had a mount, I couldn't use another.
-Discovered key binds in BC.
I think that's enough embarrassment for now. >.>